Synchronous digital systems are electronic systems that are driven by one or more clock signals. Because a clock signal is periodic, a clock signal has a narrow frequency spectrum. Synchronous digital systems generate electromagnetic energy on narrow frequency bands that include the frequency of the clock signal and its harmonics. The frequency spectrum of the clock signal and its harmonics may exceed desirable limits for electromagnetic interference at certain frequencies.
Some synchronous digital systems use spread-spectrum techniques to reduce electromagnetic interference. In a synchronous digital system that uses a spread-spectrum technique, the bandwidth of a signal is spread in the frequency domain to generate a signal having a wider bandwidth. Spread-spectrum techniques reduce the peak energy radiated by the system.
Some synchronous digital systems use spread-spectrum clocking (SSC). Serial AT-Attachment (SATA) and DisplayPort are examples of interface standards for electronic devices that require spread-spectrum clocking (SSC). As an example, a data transmission system that transmits a data signal from a transmitter to a receiver according to the SATA 2.0 standard using a spread-spectrum technique and a modulation frequency of 30-33 kilohertz (kHz) has a triangular down-spreading profile that varies between 0 and 5000 parts per million (PPM) of a unit interval of the data signal. In this example, the data rate of the data signal transmitted by the transmission system varies between 3.0 gigabits per second (Gbps) and 2.97 Gbps over time in a triangular waveform.